1. Field of the Invention
This invention relates to solids-liquid separating cetrifuges of the continuous type in which a bowl, imperforate or perforate, and a conveyor are rotated about a common axis in the same direction but at a differential speed. More particularly, the invention concerns the provision of such centrifuges with means for suppressing therein excessive torsional vibration called "chatter".
2. Description of the Prior Art
Centrifuges of the type concerned utilize speed change gearing connected between the bowl and the conveyor so that rotation of one of them by a motor causes the rotation of the other at the differential speed. The conveyor may be rotated faster or slower than the bowl but is normally rotated slower. Either the bowl or the conveyor may be directly driven by the motor, but usually it is the bowl.
Such centrifuges when operated on certain slurries such as starch or similar sticky materials develop the excessive torsional vibration of chatter at throughputs well below rated capacity. Chatter normally occurs at the natural torsional vibration frequency of the centrifuge, typically between 20 and 60 Hz, and is believed to be the result of stick-slip between conveyor and bowl when processing such materials. In the resultant torsional vibrations, the torque in the system fluctuates about the mean, typically from zero to a maximum which may approach or even exceed the maximum torque for which the machine is designed. Such great and rapid torque variations drastically shorten the fatigue life of centrifuge components subject to them, notably of the gears and of the fail-safe overload devices such as a shear pin or friction clutch. Breakage of one or the other soon occurs if chatter is allowed to persist, with consequent great expense in downtime and in replacement cost in the case of the gearing. Yet to avoid chatter, the user may have to operate at throughputs below 40% of rated capacity.
U.S. Pat. No, 3,685,722 discloses that chatter may be inhibited by introducing a resilient flexible connection of lower spring rate between rotating parts of the bowl, conveyor and gearing assembly. Chatter was so suppressed up to full rated capacity of the centrifuge by an elastomeric sleeve secured between the conveyor and its trunnion. However, location of a chatter suppressing device between rotating parts of the assembly imposes certain undesirable restrictions on the design and dimensions of the device and makes access thereto for adjustment or repair difficult.
The speed change gearing utilized in such centrifuges, such as single or multistage planetary, or "Cyclo" gearing, has, in addition to its high torque connections to the bowl and conveyor, a low torque connection to an external holder means, which may be fixed structure or rotating, such as a pinion slip device or a back drive for adjustably changing the differential speed. In the commonly used multistage planetary gearing, this external connection is from the first stage pinion, and its low torque is the torque on the conveyor connection divided by the gear ratio. The external connection normally includes the above-mentioned fail-safe device to prevent torque overload on the machine.
Because of the relatively low torque applied to it and its location external to the bowl-gearing-conveyor assembly, the external connection is an advantageous location for a chatter-suppressing device if such a device, effective in this location, can be provided. Attempts have been made before to suppress chatter by devices included in the external connection. These devices have typically been torsionally resilient elastomeric couplings, or metal springs, arranged to vibrate in response to torsional vibration of the external connection. Such devices have succeeded in suppressing chatter of the external connection to some degree, thus prolonging the life of the fail-safe device and reducing down time due to chatterinduced failure thereof. However, so far as known, they have not been effective to suppress proportionally, or event to any significant extent, chatter of the bowl-gearing-conveyor assembly, and gearing failures due to chatter have persisted at a high rate despite the utilization of such devices.